Power MOSFETs are a well known type of semiconductor transistor that may be used as a switching device in high power applications. Power MOSFETs are three terminal devices that include a source region and a drain region that are separated by a channel, and a gate electrode that is disposed adjacent the channel. A power MOSFET may be turned on or off by applying a gate bias voltage to the gate electrode. When a power MOSFET is turned on (i.e., it is in its “on-state”), current is conducted through the channel of the MOSFET between the source region and the drain region. When the bias voltage is removed from the gate electrode (or reduced below a threshold level), the current ceases to conduct through the channel. By way of example, an n-type MOSFET has n-type source and drain regions and a p-type channel. An n-type MOSFET turns on when a gate bias voltage is applied to the gate electrode that is sufficient to create a conductive n-type inversion layer in the p-type channel region that electrically connects the n-type source and drain regions, thereby allowing for majority carrier conduction therebetween.
The gate electrode of a power MOSFET is separated from the channel region by a thin oxide or other gate insulating layer. Because the gate of the MOSFET is only capacitively coupled to the channel region through the gate insulating layer, minimal gate current is required to maintain the MOSFET in its on-state or to switch the MOSFET between its on-state and its off-state. Thus, only minimal charging and discharging current is required during switching, allowing for less complex gate drive circuitry. Moreover, because MOSFETS are unipolar devices in which current conduction occurs solely through majority carrier transport, MOSFETs may exhibit very high switching speeds. The drift region of a power MOSFET, however, may exhibit a relatively high on-resistance, which arises from the absence of minority carrier injection. This increased resistance can limit the forward current density achievable with power MOSFETs. Additionally, the gate insulating layer of MOSFETs may degrade over time with use of the MOSFET, which can result in device failure and/or limit the rated operating characteristics (e.g., blocking voltage) of the MOSFET to levels that will not cause excessive degradation of the gate insulating layer.
Many power semiconductor devices are formed of silicon (“Si”), although a variety of other semiconductor materials have also been used. In particular, in high power applications, a variety of wide band-gap semiconductors have been used (herein, the term “wide band-gap semiconductor” encompasses any semiconductor having a band-gap of at least 1.4 eV) due, for example, to their high electric field breakdown strength. One such wide band-gap semiconductor is silicon carbide (“SiC”), which has a number of potentially advantageous semiconductor characteristics including, for example, a high electric field breakdown strength, high thermal conductivity, high electron mobility, high melting point and high-saturated electron drift velocity. Thus, relative to devices formed in other semiconductor materials such as, for example, silicon, electronic devices formed in silicon carbide may have the capability of operating at higher temperatures, at high power densities, at higher speeds, at higher power levels and/or under high radiation densities. Silicon carbide MOSFETs have been used as switching devices in a variety of power applications because of their ability to handle relatively large output currents and support relatively high blocking voltages.
For example, silicon carbide power double-implanted MOSFETS (“DMOSFETS”) may exhibit superior performance as compared to silicon-based MOSFETs in a number of high power switching applications. However, mass production of silicon carbide DMOSFETS may be unduly expensive because of, for example, the number of ion implantation steps. Moreover, the production process is complicated by the large number of photolithography processes that may be required, and this complexity can negatively impact overall device yields. Moreover, the channel mobility of silicon carbide power DMOSFETs may be relatively low, and thus larger chip sizes are typically required. These considerations have limited the use of silicon carbide DMOSFETs in commercial applications.
In DMOSFET devices, the channel region is located under the gate electrode, and hence current flow through the channel is in a horizontal direction (i.e., the channel defines a plane that is generally parallel to the substrate). As a result, the current only flows through a relatively small area, and hence the resistance of the channel may be relatively high. Silicon carbide MOSFETs having a trench gate structure are also known in the art. In these devices, the source is located at the top of the device and the drain is located at the bottom of the device, and hence the current flows through the device in a vertical direction (i.e., the channel defines a plane that is generally normal to the substrate). Typically, these devices have two source connections (one on either side of the gate electrode), and the channel current through these devices flows through a much larger area. This reduces the “on-resistance” of the device, which allows the device to handle much higher powers as compared to conventional MOSFETs. One specific type of MOSFET having a trench gate structure is the UMOSFET, which refers to a vertical MOSFET having a trench that generally resembles a “U” shape. UMOSFETs may operate at higher speeds and/or exhibit a lower on-resistance as compared to conventional MOSFETs.